Find out how much of this crucial bone-building vitamin your baby or toddler needs

Babies who have only breast milk should take a 400 IU supplement daily. Otherwise, be sure they're drinking at least 16 oz of D-fortified formula every day.

Toddlers and older kids need three to four servings of milk (the best source of D) daily, and talk to your doctor about giving them a multivitamin with 400 IU of vitamin D, says lead researcher Catherine M. Gordon, MD, director of the bone health program at Children's Hospital Boston. "Even though a milk label may say that 1 cup contains 100 IU, the amounts can vary," she says.